[Perf] parallel load library games

[MateoGonzalezLourido]

Game library data is now loaded asynchronously, preventing the UI from blocking during startup.

Performance impact

• Cold cache: operations that could take several seconds now complete in milliseconds.
• Warm cache: load time is roughly the same, but the UI no longer freezes while data is being fetched.

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[CommandMC]

This is largely a repeat of the second part of my comment here: #5661 (comment)

1. The UI is not driven by the Backend process, so it using synchronous code does not cause it to freeze
2. If you're gonna rewrite things to be async, at least use actual async APIs (fs/promises in this case)